1. Field of the Invention
The present invention is generally related to ceramic conductors and, more particularly, to a ceramic conductor used as an electrode in a marine cathodic protection system used to inhibit the formation of a galvanic circuit incorporating a marine drive housing and propeller.
2. Description of the Related Art
Those skilled in the art of marine propulsion systems are well aware that the use of dissimilar metals in an underwater environment can lead to the creation of a galvanic circuit with currents flowing between the dissimilar metals. As a result, the combination of a stainless steel propeller with an aluminum housing of a marine propulsion device can result in the disintegration of the aluminum housing, particularly when the marine propulsion device is used in a saltwater environment. This deleterious situation can also damage aluminum propellers when they are used in combination with steel propeller shafts and other materials having a lower electrical potential than the aluminum. This problem has been countered in the past through the use of sacrificial anodes and systems which affect the electric potential in the area around submerged materials to inhibit the galvanic circuit from forming.
U.S. Pat. No. 3,853,730, which issued to Anderson on Dec. 10, 1974, discloses a reference electrode. A marine cathodic protection device for aluminum hulls, sterndrives and outboard motors, adapted for hull installation on the bottom of a boat is described. Housings utilizing airfoil cross-section reduce drag and afford protection for the electrode surfaces.
U.S. Pat. No. 4,322,633, which issued to Staerzl on Mar. 30, 1982, discloses a marine cathodic protection system. The system maintains a submerged portion of a marine drive unit at a selected potential to reduce or eliminate corrosion thereto. An anode is energized to maintain the drive unit at a preselected constant potential in response to the sensed potential at a closely located reference electrode during normal operations. Excessive current to the anode is sensed to provide a maximum current limitation. An integrated circuit employs a highly regulated voltage source to establish precise control of the anode energization.
U.S. Pat. No. 4,357,226, which issued to Alder on Nov. 2, 1982, describes an anode of dimensionally stable oxide-ceramic individual elements. The anode consists of a plurality of individual oxide-ceramic elements of stable dimensions. The individual elements have linear cross-sectional dimensions of 2-12 centimeters on the current exit surface. These elements have a length which corresponds to 2-20 times the value of the mean linear cross-sectional dimension.
U.S. Pat. No. 4,391,567, which issued to Ciampolillo on Jul. 5, 1983, describes a corrosion preventing device for a marine propeller. It is intended for mounting in sea water on an electrically conductive propeller shaft supporting a marine propeller composed of a metal having a first galvanic potential. The propeller is fastened to the shaft by an electrically conductive propeller nut and is in electrical contact therewith. The device includes an annular washer having a generally circular periphery, composed of a metal having a second galvanic potential not greater than the first galvanic potential, with a central hole concentric with the circular axis thereof through which the shaft may fit to enable an electrical conductive mounting proximate to the propeller nut, for serving as an electrolytic cathode of the device.
U.S. Pat. No. 4,445,989, which issued to Kumar et al. on May 1, 1984, describes ceramic anodes for corrosion protection. An anode is described which is useful in corrosion protection comprising a metallic substrate having an applied layer thereon of a ferrite or a chromite, the layer having metallic electronic conductivity and a thickness of at least 10 mils.
U.S. Pat. No. 4,492,877, which issued to Staerzl on Jan. 8, 1985, discloses an electrode apparatus for cathodic protection. The apparatus is provided for mounting an anode and reference electrode of a cathodic protection system on an outboard drive unit. The apparatus includes an insulating housing on which the anode and reference electrode are mounted and a copper shield mounted below the anode and electrode to allow them to be mounted in close proximity to each other. The shield is electrically connected to the device to be protected and serves to match the electrical field potential at the reference electrode to that of a point on the outboard drive unit remote from the housing.
U.S. Pat. No. 4,528,460, which issued to Staerzl on Jul. 9, 1985, discloses a cathodic protection controller. The system for cathodically protecting an outboard drive unit from corrosion includes an anode and a reference electrode mounted on the drive unit. Current supply to the anode is controlled by a transistor, which in turn is controlled by an amplifier. The amplifier is biased to maintain a relatively constant potential on the drive unit when operated in either fresh or salt water.
U.S. Pat. No. 4,549,949, which issued to Guinn on Oct. 29, 1985, describes a marine propulsion device including cathodic protection. The lower unit of the device includes a housing having a lower portion submerged in water and defining an internal passage communicating with the water. Corrosion protection for both internal and external parts of the lower unit is provided by a sacrificial, galvanic-type anode mounted on the submerged portion of the housing and including a first or outer portion having a surface exposed to water external to the lower unit and a second or inner portion having a surface exposed to water present in the passageway.
U.S. Pat. No. 5,011,583, which issued to Tanbara on Apr. 30, 1991, describes a corrosion prevention system for a marine propulsion system. The system has a sacrificial anode for corrosion protection of the casing and includes structure whereby the propeller is electrically insulated from the casing and the sacrificial anode. The structure includes spacers made of insulating materials, spacers having insulating coatings, or insulating coatings on the surfaces of the propeller or the propeller shaft.
U.S. Pat. No. 5,298,794, which issued to Kuragaki on Mar. 29, 1994, describes an electrical anti-corrosion device for a marine propulsion device. It relates to an electrical anti-corrosion device for a marine propulsion arrangement. More particularly, the device relates to a cathodic protection arrangement which is suitable for use with an inboard/outboard propulsion unit. According to the invention, an anode and a reference electrode are housed within a housing unit which is mounted upon a propulsion unit mounting bracket. The two electrodes are arranged so that each is essentially equidistant from a point located approximately midway across the lateral width of an outboard drive unit.
U.S. Pat. No. 5,306,408, which issued to Treglio on Apr. 26, 1994, describes a method and apparatus for direct arc plasma deposition of ceramic coatings. High temperature resistant, electrically conductive ceramic compounds, such as titanium carbides and diborides, are coated onto an organic substrate. The substrate may be an organic resin matrix composite. The apparatus basically comprises a vacuum arc plasma generator, a high voltage insulated substrate holding table and a plasma channel. The plasma generator includes a vacuum chamber having a cylindrical cathode of the material to be deposited, surrounded by a ceramic insulator which is in turn surrounded by a metal trigger ring in contact with a trigger electrode.
U.S. Pat. No. 5,330,826, which issued to Taylor et al. on Jul. 19, 1994, describes a preparation of ceramic-metal coatings. A metal substrate (e.g. titanium having a calcium phosphate coating, particularly hydroxylapatite) and containing a metal such as cobalt, codeposited on the substrate by electrolyzing a cobalt salt, particularly cobalt sulfate, liquid electrolyte having a calcium phosphate material, particularly hydroxylapatite, suspended therein, employing a cobalt anode and the metal substrate as cathode is described. The particles of cobalt so codeposited with the particles of calcium phosphate material (e.g. hydroxylapatite) hold the latter particles strongly on the substrate metal. If desired, a second coating of the pure calcium, phosphate material (e.g. hydroxylapatite) optionally can be applied over the codeposited hydroxylapatite-cobalt coating. The calcium phosphate coated metal substrate of the invention, particularly the codeposited hydroxylapatite-cobalt coating, on a titanium or cobalt-chromium substrate, has particular value for application as medical implants (e.g. as hip prosthetics) and for high temperature high stressed applications.
U.S. Pat. No. 6,511,586, which issued to Nakashima et al. on Jan. 28, 2003, describes a marine organism prevention system for structures in seawater. An antifouling system for a structure exposed to seawater has an anode forming apparatus bonded to the seawater-exposed surface to be wetted with seawater of the structure via an insulating adhesive. The anode forming member is coated with an electrical catalyst film of an electrochemically active and stable electrical catalyst. A conductive member is disposed so as to be wetted with seawater. An external DC power supply has a positive terminal connected to the anode forming member and a negative terminal connected to the conductive member.
U.S. Pat. No. 7,044,075, which issued to Sica et al. on May 16, 2006, describes a marine vessel corrosion control system. The system contemplates redundant protection for a marine vessel against the effects of galvanic corrosion. The vessel is equipped with typical zinc anodes interconnected together and attached to metallic components to be protected from galvanic corrosion. A reference electrode immersed in the water provides signals to a control box representative of electrode voltage as compared to an internal stabilized voltage standard.
U.S. Pat. No. 7,064,459, which issued to Staerzl on Jun. 20, 2006, discloses a method of inhibiting corrosion of a component of a marine vessel. The method impresses an electronic current into the protected component and causes the protected component to act as a cathode in a galvanic circuit which comprises a conductor, such as a ground wire connected between the protected component and an electrical conductor which is external to the marine vessel on which the protective component is attached. The electrical conductor can be a ground wire of an electrical power cable connected between the marine vessel and the shore ground. The sea bed is caused to act as an anode in the galvanic circuit, with varying voltage potentials existing within the water between the sea bed and the protected component. The system can be a closed loop control circuit using a voltage sensed by an electrode, or an open looped circuit that provides current pulses based on empirical data.
The patents described above are hereby expressly incorporated by reference in the description of the present invention.
In corrosion protection systems that do not use a sacrificial anode, certain materials are preferred for use as an anode or electrode. These materials are selected to have a very low galvanic potential in order to prevent them from being sacrificed during normal use. In other words, unlike sacrificial anodes, the intent of this type of anode circuit is to be useful as a conductor in an electrical protective for a long period of time without requiring replacement. Typically, titanium or platinum is used as the anode in these types of systems. Since platinum is relatively expensive, a very small anode is typically used and it normally comprises a very thin layer of platinum deposited on a less expensive substrate. However, the surface of this anode must be submerged in the general vicinity of the marine drive that is being protected. As such, it is potentially subject to contact with various items, such as debris. As such, it is subject to damage. It would therefore be significantly beneficial if a non-sacrificial anode or electrode could be provided which is robust and not normally subject to damage if subjected to impact.